This invention relates to an underinflation detector and a method of correcting an output of a sensor using the same.
In order to achieve stable driving of a vehicle, a variety of systems have been developed, such as antilock brake systems and systems for detecting loss or insufficiency of inflation pressure of tires. Typically, these systems include sensors for detecting a speed, a yaw rate, and the like of the vehicle, and perform prescribed arithmetical operations to exercise auxiliary or active control over behavior of the vehicle.
The systems for detecting insufficiency of inflation pressure (underinflation) of tires fall roughly into two approaches: one that detects inflation pressure directly by a pressure sensor attached to each tire; and the other that determines change in inflation pressure from a difference in rotational speeds of tires by making use of outside diameters of the tires which changes in accordance with change in the inflation pressure. The approach that determines change in inflation pressure from differences in rotational speeds of the tires has an advantage that a rotational speed sensor attached to the tire for the antilock brake system or the like in ordinary vehicles can be used to construct the system.
Among techniques for detecting insufficiency of inflation pressure through the latter approach in which difference in rotational speeds of the tires is utilized is, for example, a method adopted by an apparatus disclosed in JP 49-85701 A in which the sum of the numbers of revolutions per unit time of the left front wheel and the right rear wheel is calculated, the sum of the numbers of revolutions per unit time of the right front wheel and the left rear wheel is calculated, and the sums of the numbers of revolutions per unit time as thus calculated are compared. This apparatus can accordingly achieve improved accuracy of detection by offsetting the difference in the numbers of revolutions per unit time between the right wheel and the left wheel which arises while a vehicle makes a turn in a curved road, and the difference in the numbers of revolutions per unit time between the front wheel and the rear wheel which is derived from the difference between the driving wheel and the wheel driven by the driving wheel.
Another technique for detecting insufficiency of inflation pressure through the approach in which difference in rotational speeds of the tires is utilized is disclosed in JP 6-92114 A, in which a control unit is programmed so as not to detect the difference in the rotational speeds of the tires in transient states of behavior of the vehicle, for example, where the vehicle drives at a very low speed, or the speed of the vehicle increases or decreases suddenly, or the vehicle undergoes a great lateral acceleration. Thus-configured apparatus can accordingly prevent a possible error in detection, thereby increasing reliability.
However, according to the underinflation detector as described above in which comparison is made between the sum of the numbers of revolutions per unit time of the left front wheel and the right rear wheel and the sum of the numbers of revolutions per unit time of the right front wheel and the left rear wheel, when tires of the two wheels at the same sides (i.e., the front and rear right wheels, or the front and rear left wheels) undergo decrease of inflation pressure at the same time, the sums for use in the above comparison would both be affected by insufficiency of inflation pressure, and thus could possibly make no significant difference as a result of the arithmetical operation. Therefore, accurate detection cannot be achieved by the above underinflation detector for some particular cases. Further, there is an increasing demand of detection with higher precision, even when only a tire of one wheel is underinflated.
On the other hand, sensors for determining stability of behavior of a driving vehicle could be affected by torque applied on attachment, change with passage of time, temperature change, and the like, so that a measurement would involve an offset. Since the sensors used in the aforementioned systems should constantly output an accurate value in order to ensure that stable driving of the vehicle will be achieved, the output of the sensors may preferably be corrected as necessary, so that no offset is involved.
In view of the circumstances as described above, the present invention has been made to address the above disadvantages. It is thus an exemplary general object of the present invention to provide means for detecting insufficiency of inflation pressure of tires of a vehicle with high reliability. Another exemplary object of the present invention is to provide means for correcting the output of a prescribed sensor by making use of the inventive means for detecting insufficiency of inflation pressure of tires of a vehicle.
An underinflation detector for a four-wheeled vehicle provided according to one aspect of the present invention includes: a driving direction sensor that detects a direction of movement of the vehicle and outputs a detected value as a first value of a parameter related to the direction of movement of a vehicle; a rotational speed sensor that detects rotational speeds of wheels of the vehicle; and a controller that determines from outputs of the driving direction sensor and the rotational speed sensor whether inflation pressure of a tire of any of the wheels has decreased. The controller obtains the first value of the parameter from the driving direction sensor, calculates a second value of the parameter from a difference between rotational speeds of the front wheels, calculates a third value of the parameter from a difference between rotational speeds of the rear wheels, calculates a rate of change of deviation of the second value from the first value with respect to change of vehicle speed by the least-squares method, calculates a rate of change of deviation of the third value from the first value with respect to change of the vehicle speed by the least-squares method, and if at least one of the rates of change of deviation exceeds a prescribed threshold, determines that inflation pressure of a tire of at least one of the wheels has decreased.
The above underinflation detector utilizes three values of a parameter related to the direction of movement of a vehicle, among which the first value is an actually measured value detected by the driving direction sensor, the second and third values are calculated from the difference between rotational speeds of the front wheels and from the difference between rotational speeds of the rear wheels, respectively. In operation, comparison is made between the first value and the second value to detect insufficiency of inflation pressure of tires of the front wheels, and comparison is made between the first value and the third value to detect insufficiency of inflation pressure of tires of the rear wheels. To be specific, for the tires of the front wheels, the deviation of the second value from the first value and the vehicle speed determined at a time when the deviation is observed are paired as a data unit, and a plurality of such data units are obtained to apply the least-squares method, by which a rate of change of deviation with respect to change of the vehicle speed is calculated. Similarly, for the tires of the rear wheels, a rate of change of deviation of the third value from the first value with respect to change of the vehicle speed is calculated by the least-squares method. Thus-obtained rates of change of the deviation each indicate a tendency of the difference of the rotational speeds between the right and left (front or rear) wheels, and thus if any of the rates of change of the deviation exceeds a prescribed threshold, it is determined that the inflation pressure of a tire of at least one of the wheels has decreased. Since detection of insufficiency of inflation pressure of tires of the front wheels and detection of insufficiency of inflation pressure of tires of the rear wheels can be made independently by comparison with actually measured values, detection can occur without fail even when the inflation pressure of only one tire among four has decreased, and even when the inflation pressure of tires of the two wheels at the same sides (i.e., the front and rear right wheels, or the front and rear left wheels) have decreased at the same time.
In the above arrangement, the deviations may preferably be modified by being weighted by normalized values of indicators of straight-ahead driving of the vehicle and taking moving averages thereof before the rates of change of the deviation are calculated with respect to change of the vehicle speed by the least-squares method.
The above modification utilizes normalized values of the indicators of straight-ahead driving of the vehicle to weight the deviations, which are in turn moving-averaged to produce a preferable determination result. In a case where the vehicle is cornering, or others, there is a high probability that a difference should arise between rotational speeds of the right and left wheels; thus, the deviations calculated in such a case are weighted, and then the moving averages of the weighted deviations are taken, so as to reduce the degree of contribution thereof to the determination of significant decrease in inflation pressure of tires. Consequently, only the difference between the rotational speeds of the wheels caused by the decrease in inflation pressure of tires during straight-ahead driving of the vehicle is relied on in detecting insufficiency of inflation pressure of the tires.
The above parameter related to the direction of movement of the vehicle may preferably be selected from the group consisting of a yaw rate, a steering angle, and a lateral acceleration.
The underinflation detector utilizing a yaw rate, a steering angle or a lateral acceleration as a parameter related to the direction of movement of the vehicle, and comparison is made between an actually measured value and calculated value for the front or rear wheel to detect insufficiency of inflation pressure of the tires. The parameter is indicative of a deviation of the direction of movement of each wheel from the straight-ahead driving of the vehicle, and thus serves to detect any imbalance in rotational speed between the front wheels or between the rear wheels. Since it is understood that such imbalance in rotational speed should be derived from decrease of inflation pressure of tires, the insufficiency of inflation pressure of tires of the front or rear wheels can be detected by using these parameters.
In another aspect of the present invention, there is provided a sensor output correcting method using the underinflation detector as above, which includes the steps of: calculating a first offset value of outputs of the driving direction sensor from a relation obtained by the least-squares method between the deviation of the second value from the first value and the vehicle speed; calculating a second offset value of the outputs of the driving direction sensor from a relation obtained by the least-squares method between the deviation of the third value from the first value and the vehicle speed; and obtaining from the first offset value and the second offset value a correction value to be added to the outputs of the driving direction sensor.
Various sensors are used to ensure that stable driving of the vehicle will be achieved, and among them is a driving direction sensor which is used to detect a driving direction of the vehicle. The above method is configured to correct a possible deviation of the output of this driving direction sensor from a true value, and to thereby obtain an adequate value from the output of the driving direction sensor. It is understood that as long as the inflation pressure of each tire has not decreased and no deviation exists in the output of the driving direction sensor, the relation between the deviation and the vehicle speed calculated by the least-squares method should exhibit a curve in which the deviation=0 if the vehicle speed=0. However, in some instances, the deviation has a specific value more than 0 even when the vehicle speed=0, and it is understood that the specific value (as an intercept of the deviation curve) indicates an offset value of the driving direction sensor.
Other advantages and further features of the present invention will become readily apparent from the following description of preferred embodiments with reference to accompanying drawings.